1.2.2. About Modules and the New Modular Class Loading System used in JBoss EAP 6

1.2.2.1. Modules

A Module is a logical grouping of classes used for class loading and dependency management. JBoss EAP 6 identifies two different types of modules, sometimes called static and dynamic modules. However the only difference between the two is how they are packaged. All modules provide the same features.
Static Modules
Static Modules are predefined in the EAP_HOME/modules/ directory of the application server. Each sub-directory represents one module and defines a main/ subdirectory that contains a configuration file (module.xml) and any required JAR files. The name of the module is defined in the module.xml file. All the application server provided APIs are provided as static modules, including the Java EE APIs as well as other APIs such as JBoss Logging.

Example 1.1. Example module.xml file

<?xml version="1.0" encoding="UTF-8"?>
<module xmlns="urn:jboss:module:1.0" name="com.mysql">
  <resources>
    <resource-root path="mysql-connector-java-5.1.15.jar"/>
  </resources>
  <dependencies>
    <module name="javax.api"/>
    <module name="javax.transaction.api"/>
  </dependencies>
</module>
The module name, com.mysql, should match the directory structure for the module, excluding the main/ subdirectory name.
The modules provided in JBoss EAP distributions are located in a system directory within the JBOSS_HOME/modules directory. This keeps them separate from any modules provided by third parties.
Any Red Hat provided layered products that layer on top of JBoss EAP 6.1 or later will also install their modules within the system directory.
Creating custom static modules can be useful if many applications are deployed on the same server that use the same third party libraries. Instead of bundling those libraries with each application, a module containing these libraries can be created and installed by the JBoss administrator. The applications can then declare an explicit dependency on the custom static modules.
Users must ensure that custom modules are installed into the JBOSS_HOME/modules directory, using a one directory per module layout. This ensures that custom versions of modules that already exist in the system directory are loaded instead of the shipped versions. In this way, user provided modules will take precedence over system modules.
If you use the JBOSS_MODULEPATH environment variable to change the locations in which JBoss EAP searches for modules, then the product will look for a system subdirectory structure within one of the locations specified. A system structure must exist somewhere in the locations specified with JBOSS_MODULEPATH.
Dynamic Modules
Dynamic Modules are created and loaded by the application server for each JAR or WAR deployment (or subdeployment in an EAR). The name of a dynamic module is derived from the name of the deployed archive. Because deployments are loaded as modules, they can configure dependencies and be used as dependencies by other deployments.
Modules are only loaded when required. This usually only occurs when an application is deployed that has explicit or implicit dependencies.